1. Field of the Invention
The present invention relates to the bleaching of deinked paper/wood pulps, and, more especially, to the bleaching of such pulps by conjointly treating same with a sulfite bleaching agent in conjunction with a particular reducing agent.
2. Description of the Prior Art
The use of bleached pulp obtained from recycled and deinked paper has become increasingly important in the paper industry, for example for the production of printing or toilet paper, or cardboard. See in this regard the text by R. P. Singh, The Bleaching of Pulp, 3rd. Ed., 324-326, 560-562.
The bleaching of deinked pulp entails use of a reducing agent, such as, for example, sodium hydrosulfite or sodium dithionite, or an oxidizing agent, such as, for example, hydrogen peroxide. Compare DE 3,118,192.
This '192 patent application describes the introduction, in the initial phase of deinking, of a reducing bleaching agent which may be a mixture of sulfite and dithionite, in a proportion of 0.1% to 2% by weight relative to the paper in the dry state, at a pH ranging from 9.5 to 12.
Dithionite, on the other hand, is applied to the deinked pulp at a pH equal to 6, a value that differs from the pH of the deinking operation and which must independently be established.